For ages i have had the same problem ,i have written to support many times, but no solve so far, my problem is that my block list is full, it has been for a looong time, it seems for blizzard accounts there are a limit to how many people you can block, you can go above it if you report someone for flame or something like that, then it blocks them despite your list being full, but i cannot block people normally which sucks, because i dont wanna report everyone i wanna block. - like if someone is spamming in the chat distracting me i would like to block him, but i dont wanna report him.
Thanks to your topic now I know there is a limit to block only 100 people. There was one person pestering me and I had to remove someone from the block list so I could block another. Now I can be at peace.
You can see how many people are blocked in battle.net settings, so you can remove someone from the list and have a free space to block another person without reporting, because even reporting a person, if you are within 100 blocked, they will not be blocked.
I hope I was helpfull.